Gun barrels are subjected to very high internal pressures and temperatures as propellant in a combustion chamber ignites and generates hot gas to provide propulsive force to a projectile. Higher temperature propellants have been developed to propel projectiles at a higher velocity from the gun barrel. Unfortunately, the higher temperatures and pressures in the gun barrels can also erode the bore surface of the gun barrel over time. In the past, an electroplated chrome finish was typically applied to gun barrels to protect the gun barrels against the increased temperatures and pressures. Over time, increasing muzzle velocity and range requirements have led to higher-temperature propellant formulations. Even electroplated gun barrels cannot weather these modern higher-temperature propellant formulations, leading to severe barrel life reductions and poor weapon performance.
In response, alternative coatings to electroplated chrome were developed to withstand the higher-temperature propellant and meet certain requirements, including the ability to withstand a higher melting point than chrome (1875° C.) and having a Young's Modulus comparable to the base material, steel. These requirements limit consideration to a few refractory metals, such as rhenium, niobium, tantalum, tungsten and molybdenum, by way of non-limiting example. Silicon nitride and other ceramic liners have similar traits and may also be considered for future weapon systems.
A major limitation with refractory metal and ceramic liners is the difficulty in machining rifling grooves. For a medium caliber weapon, typical rifling groove dimensions would be 0.5 mm+/−0.1 mm deep by 2.35 mm wide with a progressive 1 in 8 twist. These grooves may be mechanically machined, electrochemically milled, rotary forged or button rifled. In refractory metals however, machining the rifling grooves has proved difficult. Barrels lined with refractory metals may be manufactured by explosively bonding the refractory metal liner to the main barrel which can leave an uneven surface and liner that varies in thickness along the length of the barrel.
Previous attempts to machine rifling grooves in refractory metal and metal ceramic composite barrel liners have been largely unsuccessful. These attempts have used electron discharge machining (EDM), traditional machine tooling, forging, broaching, and electrochemical machining. Broaching, the traditional method used in conventional steel barrels, is extremely challenging to implement in refractory metals. Machining refractory metals with traditional machine tools typically causes rapid tool wear due to the strength and temperature resistance of the refractory metals and leaves a poor surface finish thereon. Electron discharge machining (EDM) is very slow and leaves a recast layer which negatively affects the life of the barrel. Electrochemical machining has been unsuccessful on refractory metals of interest such as tantalum and has been unsuccessful in machining some ceramic-metallic composites. Previous attempts to waterjet mill gun barrels by pointing the jet at a shallow angle to the target surface have generally led to poor depth control, rough surfaces and very slow material removal rates. Waterjet machining nozzles are typically 1″ or more in diameter and 10″ or more in length and have to be operated at right angles to the machined surface. Obviously, these nozzles and the associated operating parameters cannot be used inside small bores that are typical for small and medium caliber gun barrels.
A reliable and inexpensive method of machining the bores of refractory metal and ceramic lined barrels is needed. In addition to machining rifling grooves, a method of machining the surface of the barrel to open the diameter to a consistent dimension is also needed. Additionally, current methods do not allow the use of advanced gun barrel designs with varying rifling twist pitch angles.